It is a trend that has some investors, particularly state pension funds, increasingly concerned. They say directors who stay on a board a long time can get too cozy with management and lose their independence. It also means that the clubby domination of boardrooms by older, mainly white, men can continue, with fewer opportunities for women and minorities to get directorships.

Shareholders should "make the case for director refreshment, and vote against the reappointment of directors at stagnant boards," said Anne Simpson, corporate governance director of the California Public Employees Retirement System - the largest U.S. public pension fund with $275 billion in assets.

Many U.S. boards, Simpson said, now resemble a description given by the British governance pioneer Sir Derek Higgs who said the boardroom was the domain of the "male, pale and stale."

The Council of Institutional Investors (CII), whose members include many state pension funds, endowments and some other large asset managers, in September adopted a new policy that could make it harder for longer-serving directors to be considered "independent." That is an important definition as boards need independent members to lead committees that decide on executive compensation, monitor auditing, and set board policy in other critical areas. The policy could prompt more votes against longer-serving directors who are up for re-election in the 2014 proxy season.

Boards are doing too little to bring in fresh voices, says Michael Garland, head of corporate governance for New York City's public pension funds. "It's a board responsibility to refresh itself," Garland said.

CII though is not setting strict guidelines on director tenure or mandatory retirement ages. Past attempts by some activist shareholders to adopt formal term limits for directors have failed to get much support from shareholders.

This is not surprising given that some of the world's most legendary business leaders are still running massive companies into their 80s - in many cases they are not only active on their boards but often chair them. Among them are investor Warren Buffett, 83, Hong Kong's most famous businessman Li Ka-shing, 85, and media tycoon Rupert Murdoch, 82.

Instead, some funds are going to use the CII framework to vote against long-serving directors provided they are not perceived to be adding value, said Simpson. Garland and Simpson declined to cite specific boards they plan to target.

Institutional investors and pension funds are holding behind-the-scenes discussions with companies about the issue of board tenure, said Stephen Brown, senior director of corporate governance at fund group TIAA-CREF.

UPHILL BATTLE

Diversity and director tenure are "a focus of the pension funds this year. There happens to be support coalescing around the issue," said Chris Young, a managing director and head of contested situations at Credit Suisse.

They may well face an uphill battle. At a National Association of Corporate Directors' conference in Washington, D.C. last month, an audience of predominately older men almost all raised their hands when asked if they had been nominated as directors because they knew someone already on the board. They were then asked who was appointed to their boards due to a recruiter. "Only 5 people out of 300 put their hands up," recounted chief executive and chairman of Frontier Communications Corp (>> Frontier Communications Corp), Maggie Wilderotter, also a director at Xerox (>> Xerox Corporation) and Procter & Gamble (>> The Procter & Gamble Company).

Executive and director search firm Spencer Stuart said in its recent annual study of boards, that the average age of directors on the boards of S&P 500 companies has gone up to nearly 63 in 2013 from 60 in 2003 because of rising retirement ages and the increased recruitment of retired executives for board openings. For the first time, nearly half of the 339 newly elected directors this year had retired from their main employment.

It said that 88 percent of the boards that have a mandatory retirement age for directors have set it at 72 or older, against 46 percent 10 years ago. Twelve boards have an average age of 70 or more - including media company CBS Corp (>> CBS Corporation) (average age 72) and cable company Cablevision Systems Corp (>> Cablevision Systems Corporation) (average age 73).

Also, only 16 S&P 500 boards, or 3 percent of the total, specify a term limit for directors in their guidelines, while 20 percent of boards have an average director tenure of 11 or more years. "Investor pressure for board refreshment is growing, most notably in the form of calls to reconsider director independence as it relates to tenure," said the report from Spencer Stuart, which recruits directors.

The lack of term limits means that companies are making slow progress in getting more women and minorities on their boards - the number of women has gone up to 18 percent of all directors in 2013 from 16 percent in 2008, with 7 percent of boards still without a woman director.

"You still see boards that are all male and if seats don't turn over faster, that is going to take longer," said Lee Hanson, a vice chairman for executive recruitment firm Heidrick & Struggles.

TENURE FRAMEWORK

Currently, big fund firms typically do not suggest director term limits in their proxy voting guidelines. Executives at several asset managers including T. Rowe Price (>> T. Rowe Price Group, Inc.) and Vanguard Group said it is too soon to say if their stances on director tenure might change.

One reason for the aging of boards is that rising obligations for directors put a premium on experience. Restrictions on current top executives outside board commitments also mean that companies are more likely to turn to retired executives.

Still, the push for change is more selective and subtle than in the past. A set of proxy resolutions calling for term limits at big companies like Pfizer Inc (>> Pfizer Inc.) and United Technologies Corp (>> United Technologies Corporation) failed to get more than 10 percent support in 2006 and 2007. A similar measure at General Electric Co (>> General Electric Company) this year also made little headway.

Aware of those flops, shareholder activists have avoided calling for term limits and found other ways to make director tenure an issue in proxy contests.

At JPMorgan Chase & Co (>> JPMorgan Chase & Co.), for instance, the labor pension adviser CtW Investment Group campaigned against several long-serving directors over oversight failures such as the bank's massive "London Whale" trading losses, citing in part their long lengths of service. Two directors eventually stepped down after getting low vote totals. A third who won only narrow support, James Crown, has been a director of JPMorgan since 2004 and before that was a director of Bank One Corp since 1991. JPMorgan bought Bank One in 2004.

The U.S. approach, though, has been much more timid than the corporate governance practices in some European countries where policies suggest directors be tied to management after a certain number of years - nine years at U.K companies, for instance. CII decided that such policies could force out too many qualified directors.

"We're not saying that tenure is a limiting factor for effectiveness, we're saying this is a consideration that needs to be made," said Glenn Davis, the council's director of research.

Some worry the CII policy could bring in too many new directors. Experience has its place, said Greg Lau, former head of corporate governance for General Motors Co (>> General Motors Company) and now a consultant with executive search firm RSR Partners. Many new directors "come in not knowing the company nor the industry and it takes them a longer tenure to get to know everything," he said.

(Reporting By Ross Kerber and Nadia Damouni; Editing by Martin Howell and Tim Dobbyn)

By Nadia Damouni and Ross Kerber